1. Field of the Invention
The present invention relates to a dressing apparatus for dressing a polishing surface of a polishing table used for polishing a workpiece in a polishing apparatus, and a polishing apparatus having such dressing apparatus for polishing a workpiece such as a semiconductor wafer to a flat mirror finish.
2. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnection is photolithography. Though the photolithographic process can form interconnections that are at most 0.5 xcexcm wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small.
It is therefore necessary to make the surfaces of semiconductor wafers flat for photolithography. One customary way of flattening the surfaces of semiconductor wafers is to polish them with a polishing apparatus for performing Chemical Mechanical Polishing (CMP).
Conventionally, as shown in FIG. 43, such a polishing apparatus has a polishing table 102 having a polishing cloth (polishing pad) 100 attached to the upper surface thereof, and a top ring 104 for holding a substrate W such as a semiconductor wafer and pressing the substrate W against the polishing cloth 100 on the polishing table 102. A polishing liquid containing abrasive particles is supplied from a nozzle 106 onto the polishing cloth 100 and retained on the polishing cloth 100. The polishing cloth 100 on the polishing table 102 constitutes a polishing surface. During operation, the top ring 104 exerts a certain pressure, and the surface of the substrate W held against the polishing surface of the polishing table 102 is therefore polished to a flat mirror finish while the top ring 104 and the polishing table 102 are rotating. The polishing liquid comprises abrasive particles such as silica particles, and chemical solution such as alkali solution in which the abrasive particles are suspended. Thus, the substrates W is chemically and mechanically polished by a combination of a mechanical polishing action of abrasive particles in the polishing liquid and a chemical polishing action of chemical solution in the polishing liquid.
When the polishing process is finished, the polishing capability of the polishing cloth 100 is gradually deteriorated due to a deposition of the abrasive particles and ground-off particles removed from the substrate, and due to changes in the characteristics of the surface of the polishing cloth. Therefore, if the same polishing cloth is used to repeatedly polish the substrates W, the polishing rate of the polishing apparatus is lowered, and the polished substrates tend to suffer polishing irregularities. Therefore, it has been customary to condition the polishing cloth according to a process called xe2x80x9cdressingxe2x80x9d for recovering the surface of the polishing cloth before, or after, or during polishing.
In order to dress the surface of the polishing cloth 100 which has been deteriorated by polishing, a dressing apparatus 108 having a dressing surface is provided adjacent to the polishing table 102. In operation, the dressing surface of the dressing apparatus 108 is pressed against the polishing surface of the polishing table 102, and the dressing surface and the polishing table 102 are rotated relatively to each other for thereby bringing the dressing surface in sliding contact with the polishing surface. Thus, the polishing liquid and the ground-off particles attached to the polishing surface are removed, and planalization and regeneration of the polishing surface are conducted.
In order to primarily remove the polishing liquid and the ground-off particles from the polishing surface, the dressing apparatus 108 having a dressing surface composed of a nylon brush is mainly used. In order to primarily planalize the polishing surface by scraping off the polishing surface slightly, the dressing apparatus 108 having a diamond dresser is mainly used. The uniformity of the polishing surface which has been dressed affects greatly polishing precision of the workpiece.
However, the above-mentioned polishing apparatus has the following problems:
The first problem is that in case of polishing a substrate by a polishing table which rotates on its own axis, there is no relative movement between the polishing surface and the substrate on the rotation center of the polishing table, and hence the substrate is polished on an area of the polishing surface away from the rotation center of the polishing table. Therefore, the diameter of the polishing table 102 should be at least two times the diameter of the substrate W. Thus, the size of the polishing apparatus becomes large, and a large installation space of the polishing apparatus is required and the cost of facilities is high. This drawback is becoming significant with increasing diameter of the substrate W.
The second problem is presented by the polishing cloth 100 made of material having elasticity such as urethane. In general, a device pattern on the upper surface of the semiconductor wafer (substrate) W has various irregularities having various dimensions and steps, and is composed of different material. When the semiconductor wafer W having step-like irregularities is planarized by the polishing cloth 100 having elasticity, not only raised regions but also depressed regions are polished, and hence a large amount of material is removed from the semiconductor wafer and a long period of time is required until the semiconductor wafer is planarized. Thus, the operation cost in the polishing process is increased and irregularities of the polished surface of the semiconductor wafer are difficult to be eliminated, with the result that a high flatness of the polished surface cannot be obtained.
Further, regions on which microscopic irregularities are concentrated are polished at a high polishing rate, and regions on which macroscopic irregularities exist are polished at a low polishing rate. Thus, a large undulation is formed on the polished surface of the semiconductor wafer.
The third problem is presented by operating cost in the polishing process and environmental pollution. In order to polish the semiconductor wafer to a high degree of flatness, a polishing liquid needs to be supplied abundantly onto the polishing cloth 100. However, the supplied polishing liquid is discharged from the polishing cloth 100 at a high rate without being used in an actual polishing process. This leads to a high operating cost in the polishing process because the polishing liquid is expensive. Further, since the polishing liquid contains a large amount of abrasive particles such as silica particles, and may contain chemicals such as acids or alkalis to thus form slurry-like material, it is necessary to treat waste liquid discharged from the polishing process for thereby preventing environmental pollution. This also leads to a high operating cost in the polishing process.
In order to solve the first problem, it is conceivable that the polishing apparatus incorporates the polishing table 102 which makes a circulative translational motion (scroll motion) along a circle having a certain radius. In this case, every point on the polishing surface of the polishing table makes the same motion, and hence the diameter of the polishing surface on the polishing table may be equal to the dimension obtained by adding two times radius of gyration of the polishing table 102 to the diameter of the semiconductor wafer (substrate).
Thus, the polishing apparatus may be small in size and the installation space of the polishing apparatus may be reduced to lower the overall cost including manufacturing cost of the polishing apparatus, operating cost in the plant and cost of equipment.
In order to solve the second and third problems, it is conceivable to polish the semiconductor wafers (substrates) by the use of an abrading plate. The abrading plate comprises abrasive particles such as silica particles and a binder for binding the abrasive particles, and is formed into a flat plate. The abrading plate may be called a fixed abrasive plate. The abrading plate is attached to the upper surface of the polishing table, and the semiconductor wafer W held by the top ring 104 is pressed against the abrading plate under a certain pressure and brought in sliding contact with the abrading plate. With the sliding contact between the abrading plate and the semiconductor wafer, the semiconductor wafer is polished while the binder is broken or dissolved to thus generate fresh freed abrasive particles.
According to the above polishing process, the abrading plate is harder than the polishing cloth and has less elastic deformation than the polishing cloth, and hence only the raised regions on the semiconductor wafer are polished and undulation of the polished surface of the semiconductor wafer is prevented from being formed. Further, since a slurry-like polishing liquid containing a large amount of abrasive particles is not used, the amount of wastes discharged from the polishing process and required to be treated is greatly reduced, and hence the operating cost is reduced and environmental protection is easily carried out. Since the polishing liquid containing abrasive particles is not used, equipment for supplying the polishing liquid is not required.
In the case where the abrading plate is attached to the polishing table which makes a circulative translational motion (scroll motion) and the substrate is polished by the abrading plate, the polishing surface of the abrading plate includes a central region which is always brought in contact with the substrate, a peripheral region which is always brought out of contact with the substrate, and an intermediate region which is brought in contact or out of contact with the abrading plate. As a result, as shown in FIG. 9A, the surface of the abrading plate 16, i.e., the polishing surface 16a has a depressed region. That is, the central region A of the polishing surface suffers a large abrasion loss, the peripheral region C suffers hardly any abrasion loss, and the intermediate region B suffers an inclined abrasion loss. Even if the substrate (semiconductor wafer) continues to be polished by the polishing surface shown in FIG. 9A, the substrate cannot be planarized. Thus, it is necessary to dress the polishing surface of the abrading plate.
In such a case, if dressing of the polishing surface is conducted by a dressing tool having a circular dressing surface or an annular dressing surface smaller than the polishing surface as in the case of conventional dressing process, then the polishing surface of the abrading plate having irregularities is locally dressed, and hence it is difficult to planarize the entire area of the polishing surface. These circumstances hold true for the dressing process of the polishing surface composed of the polishing cloth attached to the polishing table which makes a circulative translational motion (scroll motion).
In order to prolong a service life of the abrading plate or the polishing cloth, it is necessary not to conduct more dressing operations than necessary. It has been customary to determine an end point of dressing operation on the basis of average dressing time obtained by operational experience.
However, since proper dressing time is changed depending on the number of times the polishing tool has been used, the number of times the polishing surface has been dressed, and the like, the proper dressing amount cannot be obtained by the above conventional method, resulting in excessive or insufficient dressing operation.
Further, the dressing surface of the dressing apparatus generally comprises a single dresser element such as a sponge, a brush, or an electrodeposited diamond plate, and it is difficult to form the dressing surface by combination of a plurality of dresser elements. A cleaning process of the polishing cloth is conducted separately from the dressing process, and cannot functionally take the place of the dressing process.
Although the dresser element on a standby condition has been immersed in a cleaning liquid such as pure water in a dresser cleaning container to prevent the dresser element from drying, cleaning of the dresser element has not been conducted. Thus, foreign matter attached to the dresser element or fragments of the dresser element are attached to the polishing surface to cause a polished surface of the substrate to be scratched, resulting in impairing the polishing performance.
It is therefore an object of the present invention to provide a dressing apparatus which can easily and reliably planarize a polishing surface having irregularities and regenerate the polishing surface efficiently.
Another object is to provide a dressing apparatus which can dress a polishing surface of a polishing table, which makes a circulative translational motion (scroll motion or circulative orbital motion) and has an advantage of a small installation space, by a dresser which requires a small installation space, and can increase a processing capability of the polishing table per unit installation area.
Still another object is to provide a dressing apparatus which can easily install a plurality of dresser elements and use dresser elements composed of different materials which can be freely combined, and can increase a dressing capability.
Still another object is to provide a dressing apparatus having a dresser element which can be cleaned in a dresser cleaning container to prevent foreign matter attached to the dresser element or fragments of the dresser element from being attached to a polishing surface of a polishing table.
According to a first aspect of the present invention, there is provided a dressing apparatus for dressing a polishing surface of a polishing table for polishing a surface of a workpiece, the dressing apparatus comprising: a dresser having a dressing surface for dressing the polishing surface, the dressing surface having an area which covers an entire surface of the polishing surface.
According to the dressing apparatus of the present invention, the entire area of the polishing surface can be simultaneously dressed by bringing the dressing surface in contact with the polishing surface and making a relative motion between the dressing surface and the polishing surface, and hence the polishing surface of the polishing table can be efficiently and uniformly regenerated over its entire area.
Material such as abrasive particles capable of dressing the polishing surface may be uniformly attached to a substantially entire surface of the dressing surface, or may be dispersively attached to the dressing surface to form a certain pattern. Thus, while the polishing table makes, for example, a circulative translational motion, the polishing surface of the polishing table can be dressed uniformly by the dressing surface.
The material capable of dressing the polishing surface may comprise particles such as diamond particles, ceramics such as SiC, or a brush. Particles such as diamond particles are attached to the dresser body by electrodeposition. A sheet to which particles such as diamond particles are attached by adhesion may be attached to the dressing surface. Further, the dressing surface may comprise an abrading plate or a polishing cloth. The brush may comprise synthetic resin such as nylon, or natural material.
According to a second aspect of the present invention, there is provided a dressing apparatus for dressing a polishing surface of a polishing table for polishing a surface of a workpiece, the dressing apparatus comprising: a dresser having a dressing surface for dressing the polishing surface; a sensor for detecting a frictional load between the polishing surface and the dressing surface directly or indirectly; and a judgement device for judging whether dressing of the polishing surface progresses to a predetermined degree on the basis of an output of the sensor.
When the polishing surface having irregularities is planarized with progress of dressing, the frictional force between the polishing surface and the dressing surface is changed, and current of a motor for rotating the dresser or the polishing table, or torque for rotating the dresser or the polishing table is changed. Thus, by detecting the change of current or torque, progress of dressing of the polishing cloth may be judged.
According to a third aspect of the present invention, there is provided a dressing apparatus for dressing a polishing surface of a polishing table for polishing a surface of a workpiece, the dressing apparatus comprising: a dresser having a dressing surface for dressing the polishing surface; a dresser cleaning container containing a cleaning liquid and having an upper open end; and a device for supplying the cleaning liquid to the container; wherein a content volume of the container is 1.5 to 2.5 times larger than a volume of an immersed part of the dresser immersed in the cleaning liquid in the container.
With the above arrangement, the dresser is cleaned in the dresser cleaning container, and foreign matter attached to the dresser element or fragments of the dresser element such as diamond particles are removed from the dresser, thus eliminating harmful influence caused by them on the polishing surface.
According to a forth aspect of the present invention, there is provided a dressing apparatus for dressing a polishing surface of a polishing table for polishing a surface of a workpiece, the dressing apparatus comprising: a dresser having a dressing surface for dressing the polishing surface, the dressing surface having a dimension in one direction which is longer than a dimension in the direction of a moving area of the polishing surface; and a moving mechanism for moving the dresser horizontally along the polishing surface.
In case of a rectangular dresser, the size of the dresser in the long side must have the dimension equal to or larger than that of the moving area of the polishing table, i.e., the dimension obtained by adding a scroll diameter to a diameter of the polishing table. However, the size of the dresser in the short side may be as small as possible, provided that the dressing condition permits. Thus, the rectangular dresser can save an installation space, compared with the circular dresser.
The horizontally moving mechanism may comprise a translation mechanism for causing the dresser to perform a translation along the polishing surface. With this structure, relative vectors on contact surface between the dressing surface and the polishing surface which makes a scroll motion are equalized over the entire polishing surface, and contact time between the dressing surface and the polishing surface is equalized over the entire polishing surface, and hence uniform dressing can be performed.
The dresser may comprise a cylindrical roller which rotates about an axis parallel to said polishing surface and has a dressing surface on its outer circumferential surface.
According to a fifth aspect of the present invention, there is provided a dressing apparatus for dressing a polishing surface of a polishing table for polishing a surface of a workpiece, the dressing apparatus comprising: a dresser having a dressing surface for dressing the polishing surface, the dresser performing no rotational motion about its own axis during dressing.
In a preferred aspect, the dressing surface comprises a plurality of dresser elements. In this case, a hard dresser element such as diamond particles is attached to the dresser body by electrodeposition, and a soft dresser element such as a nylon brush is provided on the dresser body. Then, a plurality of dressing elements including the hard dresser element and the soft dresser element are juxtaposed to form an integral dresser. This integral dresser is moved in one direction to dress the entire area of the polishing surface, thus making good use of characteristics of respective dresser elements and performing an effective dressing.
In a preferred aspect, a dresser cleaning container is provided to clean the dressing surface by supplying a cleaning liquid to the dresser cleaning container. If the dresser cleaning container is formed into an elongate shape so as to correspond to the shape of the rectangular dresser, the dresser cleaning container can save its installation space. Further, foreign matter attached to the dresser element or fragments of the dresser element such as diamond particles are removed from the dresser, thus eliminating harmful influence caused by them on the polishing surface.
In a preferred aspect, a limiting mechanism is provided to limit a lifting motion of the dresser.
In a preferred aspect, a cleaning mechanism which has an ultrasonic cleaning device or a cavitation generating device is provided on the dresser.
In a preferred aspect, a dresser cleaning container is provided for cleaning the dressing surface of the dresser using the cleaning mechanism.
In a preferred aspect, there is provided a prevention mechanism for preventing the dresser from falling out of the polishing table.
With the above arrangement, when a hard dressing surface of the dresser is pressed against the polishing surface to dress the entire polishing surface, the dresser is prevented from falling out of the polishing table, and thus from colliding with the side of the polishing table which makes a scroll motion, thus avoiding damage of the dresser or other components.
The prevention mechanism may comprise the following:
1) A guide table for supporting the portion of the dresser which projects from the polishing table is provided to prevent the dresser from falling out of the polishing table.
2) A detecting device for detecting the position of the dresser is provided to detect whether the dresser reaches an area which is located outside the polishing table and in which the dresser is prevented from falling. The detecting device may comprise a position detecting sensor attached to the dresser, e.g., the horizontal moving shaft of the dresser, or the number of pulses of a motor such as a stepping motor, or the like.
In case of 1), in order to press the polishing surface uniformly and allow the dresser to be transferred smoothly between the guide table and the polishing table, an upper surface comprising the same kind of material as the polishing table is provided on the guide table, and is flush with the polishing surface of the polishing table to equalize the amount of material removed from the polishing surface of the polishing table and the upper surface of the guide table. Further, if a polishing liquid is supplied to the polishing table, then the same polishing liquid is supplied to the guide table.
In a preferred aspect, a control mechanism is provided to control a pressing amount of the dresser against the polishing surface of the polishing table.
If a pressure of the dresser against the polishing table is controlled by a pressing force of the dresser, then in case of a soft dressing element, a small change of the pressure of the dresser against the polishing table causes the dresser element to be deformed excessively, thus making it difficult to control the pressure of the dresser. Further, an area of the polishing surface is changed with progress of a dressing process, and the pressure applied to the polishing surface is changed to cause nonuniform dressing. However, in the present invention, the positional relationship between the polishing surface and the dresser is controlled to be constant, thus preventing nonuniform dressing to be caused.
According to other aspects of the present invention, there is provided a polishing apparatus having the dressing apparatus with the above-mentioned structure.